Conventionally, a method that involves subjecting a cyclohexanone oxime to Beckmann rearrangement reaction in a gas phase in the presence of a solid catalyst is proposed as a process for producing an ε-caprolactam (for example, Japanese Patent Application Laid-Open Nos. 2-250866 and 2-275850). This reaction may sometimes have problems such that the catalyst activity is gradually decreased with reaction time due to deposition of carbonaceous material on the catalyst surface, thermal degradation of the catalyst, etc. In order to cope with the problems, methods are proposed in which a solid catalyst for the reaction is treated with an ammonia to re-activate the catalyst (see, Japanese Patent Application Laid-Open No. 5-9180; corresponding to U.S. Pat. Nos. 5,212,302 and 5,403,801) or in which a solid catalyst is treated with an aqueous solution of ammonium salt (such as ammonium chloride, ammonium sulfate and ammonium nitrate) and at least one basic material (such as ammonia, lower alkylamine, alkylamine and alkylammonium hydroxide) to provide high catalyst activity (see, Japanese Patent Application Laid-Open No. 5-170732; corresponding to U.S. Pat. Nos. 5,212,302 and 5,403,801).